Amplifier for small direct currents



Feb. 22, 1949. J. A. HlPPLE, JR, ET AL 2,462,190

AMPLIFIER FOR SMALL DIRECT CURRENTS Filed March 15, 1945 0 Grid VolzsWITNESSES: INVENTORS John 14. Hippie, Jr &.

11,; MLJuJ/u page/[d J. Grove.

Patented Feb. 22, 1949 AMPLIFIER FOR SMALL DIRECT CURREN TS John A.Hipple, Jr., Verona, and Donald J. Grove,

Pittsburgh, Pa.,

assignors to Westinghouse Electric Corpora.tion, East Pittsburgh, Pa., acorporation of Pennsylvania Application March 15, 1945, Serial No.582,936

Claims. 1

Our invention relates to amplifiers and, in particular, relates toamplifiers for extremely small direct currents.

Where it is desired to amplify extremely small direct currents such, forexample, as those with magnitudes covering a range between amperes and10- amperes which are found .in the output circuit of a massspectrometer such as described in 'Hipple Patent 2,331,189, issuedOctober 5, 1943, and assigned to the assignee of this application,difficulty arises from the fact that variations in the grid current ofthe first amplifier tube must be kept to a value which is small comparedwith the current being measured if they are not to result in seriousimpairment of the fidelity with which the output current of theamplifier follows variations in the current being measured. While it ispossible to obtain amplifier tubes having grid currents which vary soslightly with changes of voltage impressed on their control electrodesthat satisfactory operation of the amplifier :is obtainable, such tubesare of very special construction which renders them much more expensiveand less rugged than ordinary amplifiertubes nowon the market. Thelatter .tulbes, however, have too rapid a variation of grid current withgrid voltage to be satisfactoryifor measurement of the extremely smallcurrents above mentioned.

In accordance with our invention, we have discovered that by employingan amplifier circuit in which there is a negative feedback of energyfrom the output stage to the input of the first amplifier tube, suchnegative feedback being great enough so that the net voltagelamplification between the input and output terminals of the cascade isof the order of unity, and by adlusting the grid bias of the firstamplifier tube so that, when the current to be measured is zero, thefirst amplifier tube draws no grid current, it is possible to obtainsatisfactory operation while using a first amplifier tube of the lessexpensive type above mentioned in which the grid current varies at asubstantial rate with changes in grid voltage.

One object of'our invention is accordingly to provide an amplifier ofminute current, for example ranging from the order of 10-' amperes to10* amperes, which shall utilize onl ampli fier tubes having arelatively rapid variation of grid current with grid voltage.

Another object of our invention is to provide an amplifier in which thegrid current drawn by the first amplifier tube undergoes an extremelysmall variation with variation of current in the input'circuit to saidamplifier.

Still another object of our invention is-to provide an amplifier using.a first tube in which the variation of grid current with grid voltageis substantial, but which operates over a substantial range of variationof current in its input circuit with an extremely minute variation ingrid curren A further object of our invention is to provide an amplifierfor small direct currents in which only relatively inexpensive andstandard electronic amplifier tubes are employed.

Other objects of our invention will become apparent from reading thefollowing description taken in connection with the drawings, in which:

Figure l is a diagrammatic showing of the circult of an amplifier inaccordance with our invention; and

Fig. 2 is a diagram illustrating the relationship between grid voltageand grid current in an electronic tube of a type which may be used as afirst tube in an amplifier circuit in accordance with our invention.

Referring in detail to Fig. 1, reference numeral l indicatesthe firsttube of an amplifier for direct currents of Very small magnitudes suchas 10* amperes to 10- amperes. Such an amplifier may be referred to asan electrometer. The positive terminal from the source of such currentsis connected to the point 2, while the negative terminal of said sourceis connected to ground. The tube I is supplemented by any desired numberof cascade amplifier tubes such as 3, 4, and 5, the particular numbershown being, of course, merely illustrative, but the total number oftubes in the amplifier should be an even number. To give a specificillustration, the line connected to the terminal 2 may be the outputterminal of a mass spectrometer such as is shown and described in theHippie patent above mentioned, the negative terminal of the massspectrometer output circuit being grounded. The control electrode of thetube l is connected to the output circuit of some suitable tube in theamplifier such, for example, as tube 5, through a resistor -6 and asuitable bias battery 1 in a way which will be described in more detailbelow.

It will be evident to those skilled in the art that the circuitcontaining the resistor 15 and source 3 constitutes a negative feedbackto the control electrode of the tube vl.. Were this negative feedbackconnection omitted, it would be necessary to use for thesfirst tube A anamplifier tube in which the rate :of variation of the current drawnbythe control electrode of tube 1 over the range of variation of thecurrent flowing into terminal 2 from the source to be measured would besmall compared with the inflowingcurrent. To take an example, where thecurrent to be measured flowing into the terminal 2 is of the order of19- amperes, and the grid resistor connecting the terminal 2 to theoutput of the measured source of the order of 10- ohms, the yariationinvoltage :impressedon the control electrode of tube 1 would be of theorder of -0.1 volt. Under such circumstances, maintenance of constancyin the ratio of changes in output current of tube to change of inputcurrent to terminal 2 requires that the variation of control electrodecurrent of tube I should be only of the order of amperes over the rangeof control electrode voltage just mentioned. It is possible to obtain anelectron amplifier tube, for example the FP-54 tube manufactured by theGeneral Electric Company, which has a grid current variation of theorder just mentioned. However, such tubes are ex tremely delicate andexpensive.

By employing the feedback circuit through the resistor 6 and biasingsource I already mentioned as characterizing our invention, it ispossible to substitute a radio amplifier tube having a much greatervariation of grid current with grid voltage than 10" amperes when thegrid voltage varies by 0.1 volt; for example, the tube RH-507,manufactured by the Westinghouse Electric & Manufacturing Company ofEast Pittsburgh, Pennsylvania, may be used.

In accordance with our invention, the cathode of the tube I may be adirectly heated one supplied with heating current from a direct-currentvoltage source of the order of 220 volts having its positive terminal atthe point 8 and its negative terminal grounded. Current for heating theoathode of tubes I and 3 flows from the terminal 8 through resistors 9,I0, II and I2, thence through i a resistor I3 in multiple with thecathode of tube 3, then through a suitable ampere meter E4 to thecathode of tube I and thence through a resistor I5 to ground.

The anode of tube I is connected through a resistor I6 to a movable tapII on a potentiometer I8 which shunts resistor II and likewise through asuitable bias battery I9 to control electrode in tube 3. The anode oftube I is also connected through a capacitor 2i to the negative 3.

terminal of resistor I3. The anode of tube 3 is connected through asuitable resistor 22 to the positive terminal of resistor II. The anodeof tube 3 is also connected through a suitable bias battery 23 to thecontrol electrode 24 of tube 4.

The anode of tube 4 is connected through a suitable resistor 25 to thepositive terminal of resistor I0, and is likewise connected to thecontrol electrode of tube 5. The anode of tube 5 is connected to thepositive terminal 8 of the voltage source already mentioned.

The cathode of tube 4 is preferably of the indirectly heated type and isconnected to ground. The cathode of the tube 5 is preferably likewise ofthe indirectly heated type and is connected to ground through a resistor26. It is likewise connected through a suitable ammeter 21 to thepositive terminal of resistor II. A sliding tap 28 on the resistor 26 isconnected to the positive terminal of voltage source I, and thus acts toimpress a negative feedback, already mentioned, through the resistor 6on the control electrode of tube I. A screen grid electrode 29 in tube 4is preferably connected to the positive terminal of resistor II. Ascreen electrode 3I in tube 3 is likewise connected to the commonjunction of a pair of serially connected resistors 32,33, which in itform a path shunted between the positive terminal 8 already mentionedand ground.

In accordance with our invention, it is preferable to operate the tube Iwith its voltages so adjusted that when the current input to terminal 2is zero, the potential of the control electrode of tube I is adjusted tothe point where that con trol electrode draws no current.

Fig. 2 illustrates the relationship between the potential of the controlelectrode of tube I rela tive to its cathode, and the amount of currentdrawn by that control electrode from the electron emission of itscathode. In short, it is preferable to operate the tube I at the point34 corresponding to zero grid current in Fig. 2. In order to insure thatthe tube I is so operating, the terminal 2 is disconnected from thecurrent source to be measured, and a short circuit is alternately madeand broken across the resistor 6. The position of the tap 28 on resistor26 is then varied until a point is found in which the reading of ammeter21 undergoes no variation with the opening and closing of this shortcircuit. Under such conditions it is obvious that since the only currentflow between the cathode and control electrode of tube I must go throughthe resistor 6, and opening and closing a short circuit across thatresistor produces no change in the currents flowing through the outputcircuit of tube 5, the current through resistor 6 must be zero. Hence,the grid current of tube I must be zero.

The value of the bias battery I and the position of tap I! are made suchthat the relative feedback from the tap point 28 reduces the net voltageamplification through the amplifier tubes I, 3, 4 and 5 to substantiallyunity. Under such circumstances, the output resistor 26 impresses such astrong negative voltage on the control electrode of tube I, when achange in the current input to terminal 2 tends to raise or lower thepotential of that control electrode, that this tendency is reducednearly to zero. As a result, the voltage of the control electrode oftube I never departs substantially from the point 34 shown in Fig. 2,and there is correspondingly substantially no change in the controlelectrode current in tube I from the zero value indicated by that point34. There is, consequently, no substantial flow of current from grid tocathode in tube I and the entire input current to the terminal 2 fiowsto ground through the resistor 6. As a result, voltage variation on theresistor 5 is exactly proportional to current variations flowing intothe terminal 2 and output current variations through the ammeter 2'!follow, with substantial fidelity of proportion, all variations in theinput current to the terminal 2.

To give a specific example of the application of the principles Of ourinvention, the tube I may be a Westinghouse RH-5'07, tube 3 an R. C. A.185, tube 4 an R. C. A. type 657, and tube 5 an R. C. A. 6F6. Theresistors above enumerated may have the following values.

Resistor No. Megohms The bias voltage sources have the following values:7, volts; 19, 3 volts; 23, 45 volts.. Capacitor 21 is 0.1 microfarad. I

We claim as our invention:

1. In combination with an amplifier input electrical discharge tube of agrid-controlled type in which the grid current varies at a comparativelyhigh rate with changes in grid voltage, a negative feedback circuitbetween the output of said amplifier and the control electrode of saidtube adjusted to give an overall voltage amplification of the order ofunity for said amplifier, means for impressing on said grid such a biasvoltage as makes said grid current zero, and current responsive means inthe output circuit of; said amplifier.

2. In an amplifier for relatively small direct currents, an input tubehaving a grid and of a type in which grid current varies at asubstantial rate with grid voltage, a negative feed-back between theoutput circuit of said amplifier and said grid adjusted to produce anoverall voltage amplification in said amplifier of substantially unity,means for giving said grid voltage a value which makes said grid currentzero, and current responsive means in the output circuit of saidamplifier.

3. In combination with a source of relatively small direct current, anamplifier therefor comprising an input tube of the type in which thecontrol electrode current varies at a relatively high rate with thecontrol electrode voltage, a negative feedback connection from theoutput of said amplifier to said control electrode adjusted to give avoltage amplification of the order of unity in said amplifier, means tomake said control electrode current zero when said direct current iszero, and current responsive means in the output circuit of saidamplifier.

4. In an amplifier of the electronic type, an input tube of a type inwhich the control electrode current varies at a substantial rate withchanges in control electrode voltage relative to the cathode of saidinput tube, a feedback connection from the output circuit of a tubeseparated from said input tube by an even number of amplifier tubes, aresistor in said feedback connection, and means for varying the amountof voltage impressed through said resistor to make the control electrode current of said input tube zero when the input current to itscontrol electrode circuit is zero.

5. In combination with an amplifier having an input tube having acontrol electrode in which the current varies at a substantial rate withvariations in the control electrode voltage relative to the cathode, asecond tube separated from said input tube by an even number ofintervening amplifier tubes, a resistor connecting the cathode of thelast-mentioned tube to the cathode of said input tube, a feedbackconnection containing a resistor between the control electrode of saidinput tube and a tap point on the last-mentioned resistor, and means forgiving said control electrode voltage such a value as to make saidcurrent zero.

JOHN A. HIPPLE, JR. DONALD J. GROVE.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,692,904 Potter Nov. 27, 19282,190,743 Vance Feb. 20, 1940 2,224,699 Rust Dec. 10, 1940 2,354,718Tuttle Aug. 1, 1944 2,358,480 Skilling -s Sept. 19, 1944

